The present invention relates to an exhaust cleaning apparatus for cleaning an exhaust gas of an internal combustion engine, and more particularly relates to an apparatus and method of control of exhaust gas emission, employing a catalyst which is capable of reducing nitrogen oxides (NOx) even in an oxidizing atmosphere.
A prior art apparatus, such as that described in Japanese Patent Laid-open (Unexamined) Publication No. 58-5931, is intended to detect only hydrocarbon (HC) concentration and to control the air-fuel ratio in such a way that the HC concentration does not exceed a specified value in order to avoid a misfiring or flame-out, but is not intended to control a plurality of exhaust concentrations so as to improve reduction or cleaning efficiency of a catalyst, as performed in the apparatus according to the present invention.
In another prior art apparatus, such as that disclosed in Japanese Patent Laid-open (Unexamined) Publication No. 2-91443, a feedback to a stoichiometric air-fuel ratio and detection of carbon monoxide (CO) in a transient period are combined together. The apparatus, however, is devised to control adaptively a spike in an amount of CO in the transient period, but not to provide any means to improve reduction or cleaning efficiency of a catalyst.